Applicant's invention relates to semiconductor optical detector arrays and more particularly to means for biassing and reading out multicolor quantum-well (QW) detectors and detector arrays.
Most semiconductor optical detectors usefully respond to particular bands of wavelengths, or colors, that are usually determined by the detector material's composition. For example, alloys of mercury, cadmium, and tellurium are often used for detectors for the middle infrared (MIR) band and for the long infrared (LIR) band. The MIR band may be said to include wavelengths in the range from about 2000 nm to 6000 nm. The LIR band is often said to include wavelengths in the range from about 8000 nm to 12000 mm.
A multicolor optical detector is responsive to light of more than one wavelength band. Such detectors are usually formed by stacking detector sections that are responsive to particular wavelength bands and transparent to others. Light including the several wavelength bands that is incident on the stack is absorbed in one wavelength band by the first detector section, which transmits the remaining wavelength bands to the next detector section, which is responsive to one of the remaining bands.
Much work has been done recently on a wide range of electro-optic devices based on the electric-field dependence of strong absorption resonances in semiconductor quantum wells (QWs). These devices operate in a fashion that is much different from the operation of conventional semiconductor detectors like HgCdTe.
In a QW, a thin layer of one semiconductor material is sandwiched between cladding layers of a different material, with the electronic properties of the materials being such that an electric potential well (in the central layer) is formed between two electric potential barriers (in the cladding layers). The QW's small thickness, on the order of 100 .ANG., results in quantization of charge-carder motion in the thickness direction.
Also, QWs exhibit the quantum-confined Stark effect and Wannier-Stark localization, in which the wavelengths of the QW's peak optical absorptions associated with the creation of light- and heavy-hole excitons shift, respectively, to longer and shorter wavelengths in response to an applied electric field. Since these peak excitonic absorptions have finite spectral widths due to electron/hole interactions with material impurities and phonons, the transmissivity of a QW at a wavelength near a peak varies as the applied field varies. These and other aspects of QW devices are described in commonly assigned U.S. Pat. No. 5,047,822 to Little, Jr., et al.; U.S. patent application Ser. No. 08/109,550 filed Aug. 20, 1993, by Terranee L. Worchesky and Kenneth J. Ritter for "Hybridized Asymmetric Fabry-Perot Quantum Well Light Modulator" which is now U.S. Pat. No. 5,488,504; and U.S. patent application Ser. No. 08/193,979 filed Feb. 9, 1994, by John S. Ahearn and John W. Little, Jr., for "Infrared Image Converter". These three documents are expressly incorporated here by reference.
Because a single QW is so thin, devices are typically made by stacking a number of QWs, e.g., fifty, to obtain significant optical effects. Many aspects of multiple quantum well (MQW) devices are described in the literature, including C. Weisbuch et al., Quantum Semiconductor Structures, Academic Press, Inc., San Diego, Calif. (1991). The several QWs are separated by layers, such as superlattices, that form potential barriers.
In general, a superlattice is a stack of interleaved thin barrier layers and Qws in which the QWs are resonantly coupled, causing the QWs' discrete charge-carrier energy levels to broaden into minibands. Applying an electric field destroys the resonance, misaligning the energy levels in neighboring QWs and localizing them over a few QWs. This changes the optical absorption spectrum from a smooth, miniband profile to a peaked, QW-excitonic profile and blue-shifts the absorption edge.
One application of QWs is the quantum well infrared photodetector (QWIP). In the QWIP described in commonly assigned U.S. patent application Ser. No. 07/906,417 filed Jun. 30, 1992, by John W. Little, Jr., for "Miniband Transport Quantum Well Detector", which is expressly incorporated here by reference, internal photoemission of electrons from bound states in GaAs QWs into high-mobility channels in the QWIP's cladding layers increases the conductivity of the QWIP in the presence of thermal light, i.e., LIR wavelengths. The light is detected as an increase in the current flowing through the QWIP when operated at a fixed bias voltage. The characteristics of the QWIP (e.g., the peak-response wavelength, the optical bandwidth, and the electrical properties) are determined by the widths of the Qws (usually in the 4- to 8-nm width range) and the composition of the cladding layers (nominally thick layers of Al.sub.(x) Ga.sub.(1-x) As, with x ranging from 0.2 to 0.3).
That application also describes a multicolor detector in which the compositions and layer thicknesses of one QW section are selected such that the section's peak-response wavelength falls in a first wavelength band and the composition and layer thicknesses of the other QW section are selected such that the peak-response wavelength falls in a second wavelength band.
Such multicolor detectors have many uses as optical sensors and imagers, and the range of uses for such detectors is increased by fabricating them in two-dimensional arrays. As an array becomes larger, however, the complexity of the electronics needed for reading out the signals generated by the detector elements also increases. Moreover, applying the proper biasses to the individual sections of each multicolor detector element and reading the outputs of the sections are problems that have not been adequately addressed. Three-color QW detectors have been proposed but how to implement a multicolor IR QW detector array with simultaneous detection and separate readout has not.
U.S. Pat. No. 5,144,397 to Tokuda et al. discloses three or more light-responsive device components by which more than two different wavelengths can be detected. The wavelengths are selected by applying various resistances and/or voltages, but the patent does not disclose how the biasses would be applied with respect to a readout multiplexer.
U.S. Pat. No. 5,121,182 to Kuroda et al. discloses a device comprising a plurality of individually biassed photodiodes, and the biasses are selected to adjust the wavelength detected. The design of the device is such that a customized readout multiplexer would be necessary, with concomitant greatly increased cost and complexity.
U.S. Pat. No. 4,868,612 to Oshima et al. discloses a multiple quantum well device having three wells in which the layer sequence is repeated a selected number of times. Carriers in the device move by tunneling, not by using a conduction band, and the patent does not disclose how the biasses are applied. Also, the device would not provide independent simultaneous outputs for multicolor inputs.